Through-hull mooring system

ABSTRACT

A mooring system for floating platform that includes a hawspipe positioned below the sea level when the platform is ballasted for normal operations. The system also includes an outer fairlead that guides a mooring line from the floating platform to an anchoring point on the seabed. The outer fairlead is positioned relative to the floating platform&#39;s center of gravity and center of buoyancy. The outer fairlead is also positioned to be above the deballasted waterline.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Non-Prov of Prov (35 USC 119(e)) application 60/867,530 filed on Nov. 28, 2006.

TECHNICAL FIELD Background of the Invention

The invention relates to a system for mooring a floating platform in an ice flow. Mooring a platform in an ice flow requires the mooring lines to penetrate the ice flow. At least three issues are important to a mooring system designed for ice flow. First, the mooring system must not adversely effect the stability of the platform. Second, the mooring lines must be protected from the ice flow. Third, the mooring system must be accessible for inspection.

A central concept the stability of a floating platform is the metacentric height. The metacentric height is the vertical distance from the center of gravity of the platform to its metacenter. As the reaction of the mooring system loads on the platform can adversely affect the stability of the vessel, it is desirable to locate these reaction points at locations that are as close as possible to the vessel's vertical center of gravity.

Mooring lines are pulled in a played out using winches. Often, the winches are placed on or near the deck of the platform, above the waterline. In this position, the mooring lines extend from the deck to the seabed. In ice flow, mooring lines must be protected as they penetrate the ice flowing around the platform.

Inspecting a mooring system must be periodically inspected. To inspect a mooring system, an inspector must have access to the entire system. To do this, the entire mooring system must close enough to the surface that it can be raised out of the water for inspection.

These issues are satisfied with the current invention. The inventive mooring system guides mooring line through the moonpool to a water depth below the ice flow and then penetrates through the hull above lower ballasting tanks.

BRIEF SUMMARY OF THE INVENTION

The invention includes different embodiments. In a first embodiment, the invention can be described as a floating platform that includes a mono-column hull with a moonpool, a winch supported by the mono-column hull, a hawspipe with an inner end open to the moonpool and an outer end open to the exterior of the mono-column hull, wherein the outer end of the hawspipe is below the operational waterline of the mono-column hull, an outer fairlead positioned relative to the outer end of the hawspipe, and a mooring line extending from the winch and through the hawspipe and the outer fairlead.

In the first embodiment, the outer fairlead may below the center of gravity of the mono-column hull. Alternatively, the water lever the mooring line exits the hull is defined by the hawspipe. In this configuration, the hawspipe is positioned below the depth of an ice flow when the mono-column hull is ballasted for drilling operations. For inspection purposes, the outer end of the hawspipe is ideally above the waterline when the mono-column hull is deballasted. The outer fairlead may final contact the mooring line has with the hull as it extends to the seabed.

In the first embodiment, the mono-column hull may include ballast tanks that are located below the hawspipe. The mono-column hull may also include an ice flow zone and a ballast zone. When the mono-column hull includes a ballast zone, the outer end of the hawspipe may be in the ballast zone. When the mono-column hull includes a ice-flow zone, the mooring line may extend through the moonpool in the ice flow zone. Additionally, the inner end of the hawspipe is positioned adjacent the ice flow zone and the ballast zone of the mono-column hull.

In a second embodiment, the invention may include a floating platform that comprises a ballasted hull with a channel extending therethrough, which channel may be a moonpool, and ballasted hull has an operational waterline when ballasted for drilling operations and a deballasted waterline, a hawspipe connecting the channel with the outside of the ballasted hull, a winch supported by the ballasted hull, an inner fairlead connected to the ballasted hull and operationally aligned with the winch, and an outer fairlead connected to the ballasted hull and positioned between the operational waterline and the deballasted waterline of the ballasted hull, wherein the inner fairlead and the outer fairlead are in operable alignment with the hawspipe.

In the second embodiment, the outer fairlead may be positioned to transfers mooring forces to the ballasted hull as close to the center of gravity as possible. Further, the outer fairlead may be positioned below the depth of ice when the hull in a ballasted for drilling operations.

The ballasted hull of the second embodiment may include an ice flow section shaped to minimize the force from ice flowing past the floating platform and a ballasted section below the ice flow section, wherein the ballasted section is below the ice when the floating platform is ballasted for drilling operations.

In the second embodiment, the hawspipe may be above the center of buoyancy of the floating platform when ballasted for normal drilling operations.

In the second embodiment, the floating platform may include a second ballasted hull with a channel extending therethrough, wherein the second ballasted hull has an operational waterline when ballasted for drilling operations and an deballasted waterline, a second hawspipe connecting the channel of the second ballasted hull with the outside of the second ballasted hull, a second winch supported by the second ballasted hull, an second inner fairlead connected to the second ballasted hull and longitudinally aligned with the second winch, and a second outer fairlead connected to the second ballasted hull and positioned between the ballasted waterline and the un-ballasted waterline of the second ballasted hull; wherein the second inner fairlead and the second outer fairlead are in operable alignment with the second hawspipe.

The invention can also be described as a system for mooring a floating platform. In this embodiment, the system includes a mono-column hull and a moonpool extending therethrough, the mono-column hull having a deballasted waterline and a range of operational waterlines above the deballasted waterline, a mooring winch supported by the mono-column hull, an inner fairlead positioned in the moonpool of the mono-column hull, a hawspipe between the moonpool and the exterior of the mono-column hull; wherein the hawspipe opens to the exterior of the mono-column hull above the deballasted waterline, and an outer fairlead connected to the exterior of the mono-column hull adjacent to the hawspipe.

The winch of the mooring system can be cantilevered over the moonpool of the mono-column hull. When cantilevered over the moonpool, the mooring line can be lowered directly to the inner fairlead or hawspipe.

The inner and outer fairleads of the mooring system can be operationally aligned with the hawspipe. Such an operational alignment may be in a straight line.

The mono-column hull of the mooring system includes a center of gravity and a deballasted center of buoyancy, and the hawspipe may be positioned to exit the mono-column hull above the deballasted center of buoyancy. Further, the hawspipe is positioned above ballasting tanks.

The mono-column hull of the mooring system also includes a deballasted waterline, and wherein the hawspipe exits the mono-column hull above the deballasted waterline. The point at which the mooring line exits the mono-column hull is optimally below the ice-flow zone.

The foregoing has broadly outlined certain objectives, features, and advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention are described hereinafter, and form the subject of certain claims of the invention. It should be appreciated that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized that such equivalent constructions do not depart from the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention together with further objects and advantages are better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that such description and figures are provided for the purpose of illustration and description only and are not intended as a definition of the limits of the present invention. For example, although embodiments discussed herein include a mono-body hull, other hull arrangements are contemplated. For example, a semi-submersible is generally within the inventive concept.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a is a side view of a floating platform according to the invention;

FIG. 2 is a section view of the platform in FIG. 1;

FIG. 3 shows the platform in FIG. 1 during a roll/pitch motion; and;

FIG. 4 shows the platform in FIG. 1 with drilling equipment;

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a floating platform (10) for offshore drilling or production of hydrocarbons, comprising a topside (11) equipped with non-illustrated drilling and/or production equipment, and a substructure (12) which comprises a mono-column hull (13) supporting topside (11). Platform (10) is in sea (14) and has an operational waterline (15) when ballasted for drilling or production operations. Operational waterline (15) varies depending on the type of operation, weather, and sea state. When platform (10) is deballasted is has a deballasted waterline (15′). The topside (11) may comprise one or more decks, and in addition to the drilling and/or production equipment, may also comprise equipment and installations for carrying out a number of functions that are necessary in connection with a floating platform, for example, living quarters, hoisting cranes and electric generators. The mono-hull is provided with non-illustrated buoyancy tanks and ballast water tanks extending generally the length of the hull which can be filled with water in order to adjust the position of the platform in the sea 14.

Mono-column (13) includes an ice flow zone (16) shaped to minimize the force exerted on floating platform (10) by an ice-flow. In a preferred embodiment, ice flow zone (16) has a circular cross-section. The length of ice flow zone (16) is based in part on the expected thickness of the ice flow. It is also based on the expected sea state. Below ice flow zone (16), is a ballasting zone (17). Although ice flow zone (16) may include ballast tanks, most of the ballast comes from tanks in the ballasting zone (17).

Floating platform (10) includes mooring winches (18) positioned next to a moonpool (19). In the embodiment shown, mooring winches (18) are cantilevered over moonpool (19). Although winches (18) are shown on topside (11), they may be positioned in any dry location. For example, the winches may be positioned inside the hull and below the water line. Mooring winches (18) retrieve and pay-out mooring lines through moonpool (19).

The mooring lines are guided inside moonpool (19) by inner fairleads (20). Although one inner fairlead (20) is shown per mooring line, any number can be used. The inner fairlead (20) guides the mooring line as close as practical to the inner wall of moonpool (19). The inner fairleads (20) also guide the mooring lines into hawspipes (21).

Hawspipes (21) extend through mono-column hull (13) to outer fairleads (22). Hawspipes (21) are shown at a downward angle. However, hawspipes (21) may be at any angle. Hawspipes (21) are shown extending from moonpool (19) to the exterior of mono-column hull (13). In an alternative embodiment, hawspipes (21) extend between a non-ice area to the exterior. For example, mooring lines may be run inside the walls of mono-column hull (13) instead of moonpool (19). In such an embodiment, hawspipes (21) would extend from the interior area to the exterior of mono-column hull (13). Multiple mooring lines may be run through a single hawspipe (21) or each mooring line may have its own hawspipe (21).

Outer fairleads (22) guide the mooring lines to anchor points on the seabed. Outer fairleads (22) are aligned with hawspipes (21). Outer fairleads (22) are positioned below the ice flow.

It is important to be able to inspect the components of a mooring system. As such, hawspipes (21) and outside fairleads (22) are positioned above the deballasted waterline (15′).

Floating platform (10) has a center of gravity (G), a center of buoyancy (B), and a metacenter (M). These points are discussed in more detail below. Arrows indicating the weight and buoyancy of floating platform (10) are shown.

Platform (10) may be connected to the seabed by means slanting, slack moorings, or it may be held almost stationary in the sea by means of dynamic positioning, with the aid of thrusters that are controlled by an electronic control system. The forces on platform (10) are shown extending from the outside fairleads (22).

Mono-column hull (13) has a central axis (23). FIGS. 1 and 2 show floating platform (10) in the sea without any external influences. In this neutral position, the platform's central axes (23) is vertical and the center of gravity (G) and center of buoyancy (B) are aligned.

When floating platform (10) is subjected to external forces it rolls, pitches, and heaves. FIG. 3 shows the platform (10) during a roll/pitch, where platform (10) has been turned in the direction P1. The position of the waterline when the platform is ballasted for normal operations, seen in relation to platform (10), is indicated by reference numeral (15). It can be seen that portions of platform (10) to the right are above waterline (15), while portions of the platform to the left are below waterline (15). The platform's center of buoyancy is defined by the center of gravity of the water the platform displaces. Thus, when the platform rolls, the center of buoyancy, seen in relation to platform (10), moves from position B to B′. Where a vertical line from the new center of buoyancy (B′) intersects central axis (23) defines metacenter (M). On a further turning of the platform in the direction P1, the center of buoyancy will move even more. However, metacenter (M) remains at almost the same point.

The center of gravity (G) of the platform is a function of the platform's mass and the distribution of the mass. Accordingly, it is constant and independent of the platform's buoyancy and position in the sea. The center of gravity (G) does not move from central axis (23) during platform motion.

The metacentric height is an important parameter for the natural frequency of roll and pitch motion. In general, the longer the stability arm (the distance between center of gravity (G) and metacenter (M), the greater will be the moment generated by the buoyancy of the structure tending to return the structure to the neutral position. Thus, platforms with large metacentric heights have high restoring moments and tend to undergo sharp rolls and pitches, while platforms with small metacentric heights have lower restoring moments and undergo slow rolls and pitches.

FIGS. 1 and 3 show mooring lines attached to floating platform (10). Mooring lines impact platform stability by imparting additional restoring moments that add to that produced by the displacement of the center of buoyancy. As a result of the additional restoring moment, the apparent stability arm is increased.

To minimize the additional restoring moment from mooring lines, outer fairleads (22) are positioned below the center of gravity (G). Preferably, the outer fairleads (22) are between the center of gravity (G) and the center of buoyancy (B).

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

1. A floating platform comprising: a mono-column hull with a moonpool extending therethrough; a winch supported by said mono-column hull; a hawspipe with an inner end open to the moonpool and an outer end open to the exterior of said mono-column hull, wherein the outer end of said hawspipe is below the operational waterline of said mono-column hull; an outer fairlead positioned relative to the outer end of said hawspipe; and a mooring line extending from said winch and through said hawspipe and said outer fairlead.
 2. The floating platform of claim 1, wherein said outer fairlead is below the center of gravity of said mono-column hull.
 3. The floating platform of claim 1, wherein the outer end of said hawspipe is below the depth of an ice flow when said mono-column hull is ballasted for drilling operations.
 4. The floating platform of claim 3, wherein the outer end of said hawspipe is above the waterline when said mono-column hull is deballasted.
 5. The floating platform of claim 4, wherein said outer fairlead is positioned adjacent to the outer end of said hawspipe and said mooring line extends from said outer fairlead to an anchoring position without crossing any other fairleads.
 6. The floating platform of claim 1, wherein said mono-column hull includes ballast tanks below said hawspipe.
 7. The floating platform of claim 1, wherein said mono-column hull includes an ice flow zone and a ballasting zone and wherein the outer end of said hawspipe is in the ballasting zone.
 8. The floating platform of claim 7, wherein said mooring line extends through the moonpool in the ice flow zone of said mono-column hull.
 9. The floating platform of claim 8, wherein the inner end of said hawspipe is positioned adjacent the ice flow zone and the ballasting zone of said mono-column hull.
 10. A floating platform comprising: a ballasted hull with a channel extending therethrough, wherein said ballasted hull has an operational waterline when ballasted for drilling operations and a deballasted waterline; a hawspipe connecting the channel of said ballasted hull with the outside of said ballasted hull; a winch supported by said ballasted hull; an inner fairlead connected to said ballasted hull and operationally aligned with said winch; and an outer fairlead connected to said ballasted hull and positioned between the operational waterline and the deballasted waterline of said ballasted hull; wherein said inner fairlead and said outer fairlead are in operable alignment with said hawspipe.
 11. The floating platform of claim 10 wherein the channel extending through said ballasted hull is a moonpool.
 12. The floating platform of claim 10, wherein said outer fairlead transfers mooring forces to said ballasted hull as close to the center of gravity as possible.
 13. The floating platform of claim 10, wherein said outer fairlead is positioned below the depth of ice when said ballasted hull in a ballasted for drilling operations.
 14. The floating platform of claim 10, wherein said ballasted hull further comprises: an ice flow section shaped to minimize the force from ice flowing past said floating platform; and a ballasted section below said ice flow section, wherein said ballasted section is below the ice when the floating platform is ballasted for drilling operations.
 15. The floating platform of claim 14, a portion of said hawspipe is above the center of buoyancy of said floating platform when ballasted for normal drilling operations.
 16. The floating platform of claim 15, further comprising a second ballasted hull with a channel extending therethrough, wherein said second ballasted hull has an operational waterline when ballasted for drilling operations and an deballasted waterline; a second hawspipe connecting the channel of said second ballasted hull with the outside of said second ballasted hull; a second winch supported by said second ballasted hull; an second inner fairlead connected to said second ballasted hull and longitudinally aligned with said second winch; and a second outer fairlead connected to said second ballasted hull and positioned between the ballasted waterline and the un-ballasted waterline of said second ballasted hull; wherein said second inner fairlead and said second outer fairlead are in operable alignment with said second hawspipe.
 17. A mooring system comprising; a mono-column hull and a moonpool extending therethrough, said mono-column hull having a deballasted waterline and a range of operational waterlines above the deballasted waterline; a mooring winch supported by said mono-column hull; an inner fairlead positioned in the moonpool of said mono-column hull; a hawspipe between the moonpool and the exterior of said mono-column hull; wherein said hawspipe opens to the exterior of said mono-column hull above the deballasted waterline; and an outer fairlead connected to the exterior of said mono-column hull adjacent to said hawspipe.
 18. The mooring system of claim 17, wherein said mooring winch is cantilevered over the moonpool of said mono-column hull.
 19. The mooring system of claim 17, wherein said inner and outer fairleads are operationally aligned with said hawspipe.
 20. The mooring system of claim 17, wherein said mono-column hull has a center of gravity and a deballasted center of buoyancy, and wherein said hawspipe exits said mono-column hull above the deballasted center of buoyancy.
 21. The mooring system of claim 20, wherein said mono-column hull has a deballasted waterline, and wherein said hawspipe exits said mono-column hull above the deballasted waterline.
 22. The mooring system of claim 21, wherein said mono-column hull include an ice-flow zone; and wherein said hawspipe exits said mono-column hull below said ice-flow zone.
 23. The mooring system of claim 22, wherein said mono-column hull further includes ballasting tanks positioned below said hawspipe. 